The performance of a reentry type turbine is critically dependent on maintaining tight clearances in the region between the turbine rotor, the nozzle and the collector duct assembly that supports the exhaust duct. The reason is that the just defined region is at a pressure level intermediate to those of the inlet and exhaust pressure. Because the pressure in this region is higher than the contiguous zone surrounding the region, gas leaks from the region to the lower pressure zone. This leakage represents wasted energy.
In most applications the turbine assembly is required to handle hot gas and also must be able to accept the hot gas from a cold start. Invariably, this type of operation leads to a thermally induced differential growth of the turbine rotor and mating support components which affects the clearance between the rotor and the mating stationary support components of the housing. Because of this thermally induced differential growth, the clearances between the turbine's mating components when the turbine assembly is cold must be large enough to allow for the thermally induced differential growth that arises during the period from cold start to normal operating temperature. These large clearances are definitely undesirable from a turbine performance stand point.
The problem defined above has been recognized in part in the Kolthoff, Jr. U.S. Pat. No. 3,039,737 ('737) in respect of thermally induced radial growth of a turbine rotor. The '737 patent provides a means for thermally varying the internal diameter of the shroud surrounding the rotor to compensate for thermally induced changes in the diameter of the rotor. The invention to be described hereinafter accomplishes not only a change in the radial clearance as a consequence of thermally induced radial growth, but also maintains a controlled clearance between the nozzle and the collector duct assembly in response to thermally induced changes in the axial location of the rotor. The maintenance of the aforementioned clearances being dependent on the actual changes in the turbine rotor dimensions and does not need as the '737 patent requires a means that attempts to heat surrounding turbine cooperating support structures to the same temperature as the rotor in order that the housing and rotor experience the same thermal growth. It should also be kept in mind that the '737 patent assumes that all cooperating support structures will respond in a uniform thermal growth manner which in practice tends not to be the case.